1. Field of the Invention
This invention relates to a multimedia communication apparatus represented by an AV (audiovisual) communication apparatus, such as a video telephone apparatus, a television meeting system or the like, in which mutual communication can be performed by multiplexing multimedia information comprising voice information, video information, data and the like.
2. Description of the Related Art
Recently, communication services via an ISDN (integrated services digital network) have started to be practically used, and AV services, such as video telephones, television meeting systems and the like, using such a digital network have attracted notice. Service provisions for AV services, protocol provisions, multimedia multiplex frame structure provisions, encoding methods of moving-picture information and the like have been announced as CCITT (Comite Consulatif International Telegraphique et Telephonique)'s recommendations H. 320, H. 242, H. 221, H. 230, H. 261, and the like. In H. 221, frame structures in AV services on channels having bit rates of 64 kbps (kilobits per second)-1920 kbps are provided.
FIG. 12 shows the frame structure in H. 221 on a single channel having a bit rate of 64 kbps. In FIG. 12, numerals 1 -8 indicated in the abscissa represent bit numbers, and numerals 1-80 indicated in the ordinate represent octet numbers. One frame comprises 80 octets. In FIG. 12, frame synchronization, multiframe synchronization, the function of monitoring communication quality, notification of alarm information, and the like are controlled by FAS's (frame synchronizing signals).
FIG. 13 is a diagram showing the bit assignment of FAS's in 1 multiframe=8 submultiframes (1 submultiframe=2 frames). BAS's (bit-rate allocation signals) assign the capability of terminals and actual allocation of bit rates to respective media within frames, and perform various kinds of controls and notification. As shown in FIG.2, BAS's are transmitted in even frames, and corresponding error-correcting bits are transmitted in odd frames.
In H. 242, communication procedures, such as a capability-information exchange sequence, a mode switching sequence and the like using BAS's on in-channel between AV terminals are provided. In H. 320, system aspects for the entirety of AV services are provided. In H. 230, transmission-frame synchronization or various kinds of controls and notification requiring an urgent response are provided as additional information to functions necessary for AV services. In H. 261, methods of encoding/decoding moving-picture information in bit rates of p.times.64 kbps (p=1-30) are provided.
FIG. 14 shows a basic sequence in performing multimedia communication of images, voice, data (all kinds of user's information other than images and voice) according to the above-described recommendations.
First, by starting a multimedia communication apparatus which is intended to start communication (hereinafter termed a calling-side apparatus), a first channel is established in step 141. In the case of the ISDN, this corresponds to a call-setting sequence in the D-channel- Subsequently, in step 142, frame synchronization is established by retrieval/detection of an FAS and transmission/detection of A-bit=0 on the channel set in step 141 (corresponding to the B-channel, the H-channel or the like in the ISDN).
After establishing synchronization, in step 143, the capability of the communication partner's apparatus is determined by a capability-information exchange sequence by transmission, reception and detection of a capability BAS. Subsequently, in step 144, multiplex allocation of multi-media information on the first channel is determined, a mode switching sequence by transmission/reception of a BAS command is performed, and multimedia-information multiplex communication is started.
In practice, there is a case in which only voice information can be communicated, or a case in which only voice information is communicated until an additional channel is established.
The process then proceeds to step 145, and further to step 146 if the communication partner's apparatus has a capability of providing an additional channel, and the additional channel is established. Subsequently, in step 147, processing for establishing frame synchronization and multiframe synchronization, and synchronization with the first channel, is performed utilizing retrieval/detection of an FAS in the additional channel, and an A-bit. The process then returns to step 145, where it is determined whether or not an additional connection is further required.
If an additional channel is not required, in step 148, an operational mode suitable for the utilization of all the channels is determined, a mode switching sequence by transmission/reception of a BAS command is performed, and multimedia multiplex communication is performed.
There is a case in which the mode switching sequence by a BAS command to be performed in step 148 is performed immediately after step 144, ant a mode switching sequence is performed every time a channel is added whenever necessary. In the above-described recommendations, however, there is no particular provision about whether the start of an operation for establishing an additional channel must be performed by the calling-side apparatus or by the reception-side apparatus, and what a specific trigger for that operation is. Furthermore, there is no particular provision either about whether an additional channel must be unconditionally established if each of the two apparatuses has the capability of providing an additional channel, or whether an operation for establishing an additional channel must be started by some particular trigger.
In the above-described conventional approach, however, in an apparatus which unconditionally sets an additional channel after it has been determined that the communication partner's apparatus also has the capability of providing an additional channel by a capability-information exchange sequence, while the user need not intentionally perform an operation to set an additional channel, an additional channel is automatically set even if it is unnecessary, and therefore an unnecessary network tariff is charged without the user acknowledging it.
In an apparatus in which an additional channel is always started by an operation of the user, the user will know he must always be alert to the need for this operation, and the operation itself can be troublesome.
In addition, in the conventional approach, sufficient care has not yet been taken &bout when and in which conditions video information must be transmitted. Accordingly, transmission is unconditionally started even if a transmission rate allocatable to video information is insufficient, or an additional channel allowed by a capability-information exchange sequence is unconditionally set. As a result, the additional channel is in a vacant state because transmission of video information is not immediately started, whereby an unnecessary network tariff is charged. Furthermore, in the transmission of video information, a trigger to start transmission at the calling side and the reception side is not considered. Hence, it may happen that transmission of video information is immediately started also at the reception side, or although it is possible to change the setting of whether or not video information must be transmitted, the setting cannot be easily changed.